Avalanche photodiodes (APDs) are photodiodes that can generate a relatively large electrical current signal in response to the receipt of a relatively low-power optical signal (i.e., APDs have high responsivity). A typical APD includes a first semiconductor layer (an absorption layer) in which light energy is absorbed to create free charge carriers and a second semiconductor layer that contains a multiplication region, which is a region in which free charge carriers multiply to create a detectable electrical current.
In operation, an APD is “biased” by applying a voltage across the APD to create a high electric field. Free electrical carriers generated in the absorption layer are injected into the multiplication region. In the multiplication region, the free carriers are accelerated to a velocity that enables them to create more free carriers through a process called “impact ionization.” The resultant additional free carriers are also accelerated by the electric field and create even more free carriers, and so on. This process is referred to as avalanche multiplication, and is responsible for the high responsivity of an APD.
An APD is characterized by a breakdown voltage. The breakdown voltage of an APD is the voltage at which the APD is sufficiently electrically-biased to conduct a large current arising from a self-sustaining avalanche process that occurs in its multiplication region—even in the absence of continuous optical power. Typically, an APD is operated in one of two modes. In linear mode operation, the APD is biased slightly below the breakdown voltage, with consequent gains being modest and substantially proportional to the intensity of the incident light. In Geiger mode, the APD is biased slightly above its breakdown voltage with the specific intent of generating very large avalanches that allow for the detection of single photons.
Breakdown voltage is a critical device parameter for an APD. Unfortunately, APD breakdown voltage is highly dependent upon its layer structure and the properties of the semiconductor layers of which it is formed. These factors have historically been extremely difficult to control from wafer to wafer, fabrication run to fabrication run, and even across a single wafer within a run. Local variations in process parameters, such as temperature and/or gas flow, can lead to significant variations in the breakdown voltage across a wafer of APD structures.
While it is possible to adjust for a variation in expected breakdown voltage for a single APD, it can be quite complex and costly to measure and compensate for individual breakdown voltages within an array of such devices. For applications in which a plurality of APDs is required, such as in imaging sensors, position sensors, etc., the impact of breakdown voltage variation is, therefore, particularly damaging. Such variation necessitates costly inspection methods, complex control circuitry, increased cost, and often a degradation of the performance of the APD array.